The industrial manufacture of sodium chlorate is carried out mainly by electrolysis of a sodium chloride solution. The industrial sodium chloride, industrial water and raw materials used during manufacture almost always contain alkaline-earth cations such as calcium and magnesium. Such cations deposit on the cathode; they do so in the carbonate state when graphite anodes are used, and substantially in the hydroxide state when metallic anodes are used. The rate of formation of cathodic crusts increases with increasing operating temperature and increasing current density, which is characteristic of the use of metallic anodes.
These deposits, which have a compact texture, firmly adhere to the cathode. They tend to insulate the cathode electrically, and hence lead to an increase in the total voltage at the terminals of the cell if a constant electrical current density is to be maintained.
Thus, the presence of alkaline-earth cations in the electrolyte used for the preparation of sodium chlorate leads to an increase in specific energy consumption, on the one hand, and to the necessity of subjecting the cathodes and cells to periodic cleaning, on the other, with the frequency of such cleanings increasing with increasing operating temperature and increasing current density.
Such cleanings must be carried out very frequently when anodes consisting of a metallic support and a superficial coating are used; such anodes are of major interest precisely because they make it possible to carry out an electrolysis at a high temperature and high current density.
The technique currently used for carrying out this periodic cleaning of the cathodes consists in stopping the electrolysis, emptying the cells, cleaning the cathodes by acid treatment, rinsing the cell, replacing the electrolyte and restarting the cell. Hence this is an expensive technique due, in particular, to the interruption of operations which it involves.
The acid treatment generally used consists in a treatment with hydrochloric acid, carried out with dilute hydrochloric acid whose concentration is less than 10% by weight, to avoid corrosion of the steel cathodes and other steel components of the cells. The addition of a corrosion inhibitor to the hydrochloric acid bath is recommended.